Static semiconductor memories are often referred to as SRAMs ("static random access memory") because (unlike DRAMs or dynamic random access memories) they do not require periodic refresh signals to retain their stored data. The bit state in an SRAM is stored in a pair of cross-coupled inverters which form a circuit termed a "flip-flop." The voltage on each of the two outputs of a flip-flop circuit is stable at only one of two possible voltage levels because the operation of the circuit forces one output to a high potential and the other to a low potential. Flip-flops maintain a given state for as long as the circuit receives power, but they can be made to undergo a change in state (i.e., to flip) upon the application of a trigger voltage of sufficient magnitude and duration to the appropriate input.
As SRAMs have evolved, they have undergone an increase in density. Most of the increase in density has been due to the use of smaller linewidths. As linewidths have begun to shrink below one micron, artisans have noticed that problems with stepper lens astigmatism make it extremely difficult, if not impossible, to maintain constant linewidths over a printing field. Frequently practitioners have noticed that lines in one particular direction exhibit greater dimensional consistency than lines in other directions.
Typical SRAM cells utilize gates formed by the above-mentioned lithographic processes. The above-mentioned variation in linewidth affects gate size, and thus device performance, and ultimately cell performance.
Another problem confronted by circuit designers is the manner in which one of the access transistors is connected to the appropriate pull-down transistor. The connection often requires a nitride overlayer on the gate and additional processing steps.